A typical low-slope roofing system consists of three components: a structural deck, a thermal insulation barrier and a waterproofing membrane, which consists of reinforcing fibers or fabric sandwiched between two sheets of flexible matrix. The matrix material is either asphalt- or polymer-based. There are essentially two classes of polymer-based roofing membranes: thermosets and thermoplastics. Thermoplastics reversibly soften when heated, whereas thermosets do not. Thermoplastics encompass thermoplastic polyolefins (TPOs).
A typical TPO is a melt blend or reactor blend of a polyolefin plastic, typically a polypropylene polymer, with an olefin copolymer elastomer (OCE), typically an ethylene-propylene rubber (EPR) or an ethylene-propylene-diene rubber (EPDR). Conventionally, a suitable roofing membrane exhibits the following properties: sufficiently strong enough to withstand stresses and to accommodate deck movement. Examples of commercially available TPO membranes include SURE WELD™ (Carlisle Inc.), GENFLEX™ (Omnova Solutions, Inc), ULTRAPLY™ (Firestone Building Products) and EVERGUARD TPO™ (OAF). Typically, these membranes are fixed over a roof having insulation material placed thereon.
Problems with these membranes are that they have a rigid feel, tend to hold their shape, and do not relax quickly. The stiffness characteristics affect the membrane installation process. As such, they are generally not pliable to conform to the shape of detailing profiles (e.g. around HVAC and skyline windows) where detailing works are required for waterproofing.
Conventional reinforced TPO membranes are manufactured with a reinforcing scrim encapsulated between two layers of TPO compounds. In one example, the TPO sheet is reinforced with a polyester fleece reinforcement layer. Two TPO membranes are typically sealed by hot air heat-welding or seamed together using an adhesive. The hot air melts the polymer at the seam and the two strips of membrane become fused and bonded with gentle pressure. Another desirable characteristic of TPO membranes is the high heat seal strength. There are two sets of forces constantly working to damage the roofing seams. Wind uplift attempts to peel the seams apart. Building movement attempts to pull the seams apart. High seal strength provides high wind resistance for the roof and long life waterproofing. A conventional Ziegler-Natta catalyzed TPO has a bimodal or broad molecular weight distribution. The low molecular weight oligomers migrate to membrane surface preventing welding or deteriorating the heat seam strength. A TPO with a narrow molecular weight distribution eliminates surface blooming and provides high heat seal peel strength.
In contrast, ethylene propylene diene monomer (or terpolymer which is simply a product consisting of three distinct monomers) (“EPDM”) has been in use on roofs in the USA since the 1960's and is one of the most common types of low-slope roofing materials. This is because it is relatively inexpensive, simple to install, and fairly clean to work with when compared to conventional built-up roofs. In addition, due to its superior stretchability to conform to roof corners and angle changes, Nonreinforced EPDM is widely used as a roof covering for most motor homes and recreational vehicles. In one specific application, TiO2 is added to produce a white EPDM. However, compared to white TPO, white EPDM typically does not stand up to the weather because it caulks, loses its color, shrinks and does not lay flat.
EPDM roofs are typically single-ply membranes either a single nonreinforced EPDM layer or a sandwich of top, bottom TPO layers and a reinforcement within. EPDM is classified as a thermoset material which means it is either fully-cured prior to being installed or that it cures during natural weathering after installation. EPDM is a rubber material whose principal components consist of the compounds ethylene and propylene. A flexible rubber matrix forms when a small amount of diene is added to the mix. EPDM is available reinforced or unreinforced with both commonly used; it's also available in either a cured (vulcanized) or uncured (non-vulcanized) state. Vulcanized EPDM is the most common with non-vulcanized often used for flashing purposes.